In any time-division multiplex data communication network where the exact number of the participants is unknown, there must be found some contention-resolution means to deal with the possible situations of messages being sent from multiple participants at the same time, and, as a result, arriving garbled.
Currently, there are various ways to accomplish contention-resolution, each suffering from one type of deficiency or another. For example, in the classic ALOHA network, a participant transmits its message and listens at the same time (or with some pre-defined delay). If it hears its own message garbled, then it waits a random length of time before retrying. Slotted ALOHA improves on the performance of the basic ALOHA network by requiring that network participants transmit on pre-defined time boundaries. This approach requires that one of the participants provide a timing marker for the rest.
Carrier Sense Multiple Access (CSMA) networks are an improvement over ALOHA networks in that participants listen for another transmission before attempting to transmit themselves. In Carrier Sense Multiple Access-Collision Detect (CSMA-CD), one version of which is IEEE 802.3, the sender senses to determine whether the voltage level on the medium is greater than that generated by the sender itself. If so, then the sender will follow a binary exponential back-off scheme until it can send its message in the clear.
Multiple Access with Collision Avoidance (MACA) techniques use the broadcast of Ready-to-Send (RTS) and Clear-to-Send (CTS) messages to minimize the probability of collisions. However, when the collisions do occur, a binary exponential back-off is used to determine when a retry should occur.
All of the above-mentioned contention-resolution ways have two factors in common. First, each participant (remote) in the network is responsible for determining if a collision has occurred. Second, the participants retry at random times after the collision, either over a fixed period of time or over a period of time that continually expands, possibly exponentially. Potential problems associated with these are: one, each participant in the network must be capable of detecting collisions, even while the participant is transmitting its own message. In radio-frequency based networks, this may be extremely difficult, if not impossible; two, if the network is lightly loaded and the time within which participants may retry is fixed, there may be significant length of time wasted. If on the other hand, the network is heavily loaded, collisions may be excessive and no messages get through. If the retry time is allowed to expand, then the delay time may, again, become excessive. These problems may be particularly acute when registering the participating units into the network at the start of the network operation, when the number and the location of the participating units are unknown.